In designing data transmission systems, especially cellular radio systems, two essential aspects have to be especially taken into account. Firstly, the required geographical area is to be covered at low costs. Secondly, information is to be transmitted with the smallest possible number of transmission errors.
It is typical of digital data transmission systems in which speech information is transmitted that they apply speech coding, to change analog speech into a digital form. For instance in a fixed telephone network, analog speech is changed into a digital signal the bit rate of which is 64 kbit/s. In many data transmission systems, for instance in cellular radio systems, this transmission rate is impractical since it requires too much transmission capacity. The speech coding used in these systems is thus implemented, if possible, using a transmission channel and a bit rate as small as possible.
Speech coding techniques are constantly studied, and thus, for instance in designing cellular radio systems, the development of technology has been taken into account by making an allocation in the system for the use of better speech coding algorithms. This is the case for instance in the GSM (Groupe Special Mobile) system, in which is specified a so-called half rate codec, the bit rate of which is half of the bit rate of a so-called full rate codec. With the use of a half rate codec, the capacity of the system can thus be increased, since the amount of bits to be transmitted is half of what it used to be. For instance, in TDMA (Time Division Multiple Access) systems, it is thus possible to transmit a double amount of coded speech in the same amount of time.
In constructing a radio telephone system, attempts are thus made to produce a desired coverage area at lowest possible cost. In considering the location of the base stations of the system, both the required traffic capacity and the produced cell size are taken into account. In areas where the expected amount of traffic is great, cell sizes are small, whereas in sparsely populated areas where the required traffic capacity is small, attempts are made to produce as large cells as possible. Transmission capacities used at base stations and terminal equipment thus increase. In some cases, where it is uneconomic to construct a dedicated base station, it is possible to use a separate repeater, which receives signals arriving from a base station and repeats them within its own area. Costs can thus be reduced to some extent, however not significantly.